1. Field of the Invention
The present invention relates to bottom gate thin film transistors (TFTs) for use in liquid crystal display (LCD) devices and organic electroluminescent display devices. The invention also relates to methods of producing the TFTs.
2. Description of the Prior Art
Hydrogenated amorphous silicon (a-Si:H) TFTs are widely utilized for switching elements in liquid crystal display devices since they can be accurately fabricated on inexpensive glass substrates by a low temperature process.
There are two types of structures for a-Si TFTs, a top gate structure and a bottom gate structure. The top gate structure has a drawback that the interface between an a-Si:H thin film, which is later formed into the channel, and a gate insulating film is often contaminated during the fabrication. In contrast, the bottom gate structure is advantageous in that, since the a-Si:H thin film and the gate insulating film are successively produced without being exposed to atmosphere, the performance of the TFTs is not degraded by the contamination and the electron mobility is larger than the top gate TFTs. Thus, for switching elements in liquid crystal display devices or the like, bottom gate TFTs are advantageous. Bottom gate TFTs have two major types, etch stopped type and channel etch type (also referred to as back channel etch type). Channel etch type TFTs require less photomasks in the fabrication process than etch stopped type TFTs. This makes channel etch type bottom gate TFTs advantageous in terms of manufacturing cost, and therefore, channel etch type bottom gate TFTs have increasingly been favored recently.
FIGS. 7A to 7F show the production process of a channel etch type bottom gate TFT, each of the cross sectional views illustrating a step of the production process. With reference to these figures, a fabrication procedure of a channel etch type bottom gate TFT is described below.
A gate electrode 62 is deposited on an insulating substrate 61 to a thickness of 200 nm by sputtering and thereafter patterned into an island by photolithography and etching (FIG. 7A). Typically, the gate electrode 62 is made of an aluminum film, or a layered film made of an aluminum film and a film of a metal having a high melting point, such as titanium.
Subsequently, an SiNx film, which serves as a gate insulating film 63, is deposited to a thickness of 300 nm by plasma enhanced chemical vapor deposition (PECVD), and thereafter, without exposing the surface to atmosphere, an a-Si:H film, which serves as a high resistivity semiconductor film 64, is deposited to a thickness of 200 nm by PECVD. Then, an n+ a-Si:H film, which serves as a low resistivity semiconductor film 65, is deposited to a thickness of 20 nm by PECVD. Thereafter, stacked layers of the high resistivity semiconductor film 64 and the low resistivity semiconductor film 65 are processed into an island by photolithography and etching (FIG. 7B).
Subsequently, a source/drain electrode metal 66 is deposited by sputtering (FIG. 7C). Thereafter, a resist 67 is coated (FIG. 7D), and an opening is formed by photolithography and etching in a portion thereof which is located above the channel region. Thereafter, the low resistivity semiconductor film 65 is etched with the use of the same resist pattern to form a back channel (FIG. 7E).
This step is generally referred to as a channel etch step.
Subsequently, in order to protect the back channel exposed by the etching, a silicon nitride film serving as a passivation film 68 is deposited to a thickness of 300 nm by CVD. Finally, an opening 69 for connecting a pixel electrode is opened in a predetermined position in the passivation film 68 by photolithography and etching. Thus, a TFT is completed.
However, the foregoing prior art method has at least the following problems. In the prior art method, due to the fact that the etching selective ratio between the low resistivity semiconductor film 65 and the high resistivity semiconductor film 64 is small, overetch is caused in the channel etch step and thereby the high resistivity semiconductor film 64 is considerably etched in addition to the low resistivity semiconductor film 65 (FIG. 7E). When such overetch occurs, hydrogen in the back channel, which is composed of an a-Si:H film, is lost, and etching damage is caused by which the film uniformity with respect to the vertical orientation of the film is degraded. The etching damage deteriorates various TFT characteristics. For example, the field effect mobility of the TFT decreases to about half.
To reduce the etching damage to the back channel, if the film thickness of the high resistivity semiconductor film 64 is in the time required for the film deposition correspondingly increase reducing efficiency in production. On the other hand, if the deposition rate is increased and the deposition time is thereby reduced, film quality is degraded. In other words, both of the approaches have problems; the former increases fabrication cost due to an increase in production tact time, whereas the latter degrades production yields and TFT characteristics. Therefore, the prior art method cannot achieve efficient production of bottom gate TFTs which sufficiently function in high resolution display devices in which moving pictures are displayed.
Accordingly, it is a first object of the present invention to provide a method of producing a bottom gate TFT that solves the foregoing and other problems in the prior art. It is a second object of the invention to provide a liquid crystal display device and an organic electroluminescent display device to which the manufacturing method of the invention is applied. These and other objects are accomplished, in accordance with the present invention, by the following embodiments which include a range of aspects.
According to a first aspect of the invention, there is provided a method of producing a bottom gate thin film transistor, comprising the steps of:
forming a gate electrode on an insulating substrate;
forming a gate insulating film over the gate electrode;
forming a first semiconductor thin film for a channel over the gate insulating film;
forming a second semiconductor thin film for a source and a drain over the first semiconductor thin film;
processing stacked layers of the first semiconductor thin film and the second semiconductor thin film so as to be formed into an island;
subsequent to the step of processing stacked layers, depositing a source/drain electrode metal over the stacked layers of the first semiconductor thin film and the second semiconductor thin film;
etching a region of the deposited source/drain electrode metal, the region being located above the channel, in the depth direction to expose the second semiconductor thin film, whereby a source electrode and a drain electrode are formed; and
etching away the exposed portion of the second semiconductor thin film in the depth direction with the use of a non-ionic excited species to form a channel.
In this fabrication method, non-ionic excited species are used in the etching of the second semiconductor thin film for the source and drain (so-called channel etch). The use of non-ionic excited species reduces etching damage to the back channel because the excited species are not accelerated by electric field. Therefore, the production yield increases, and reliability in product quality of the produced channel etch bottom gate TFTs remarkably improves.
According to a second aspect of the invention, the fabrication method of the first aspect may be such that the non-ionic excited species is generated by bringing molecules of a chemical substance into contact with a metal heated by electric resistance heating to decompose the molecules of the chemical substance.
This fabrication method utilizes a catalytic CVD technique and makes it possible to produce a large amount of non-ionic excited species with simple equipment. In addition, in this method (contact-decomposition reaction method), few ionic excited species are generated. It is noted that the molecules of the chemical substrate herein are meant to include molecules composed of a single element, such as H2 molecules.
According to a third aspect of the invention, the method of the second aspect may be such that the non-ionic excited species is a radical.
In non-ionic excited species, radicals have large energy. Accordingly, good etching efficiency can be obtained. Therefore, the use of radicals is preferable.
According to a fourth aspect of the invention, the method of the third aspect may be such that the metal is selected from the group consisting of tungsten, tantalum, molybdenum, vanadium, platinum, and thorium, or is an alloy comprising at least two metals selected from the group consisting of tungsten, tantalum, molybdenum, vanadium, platinum, and thorium.
These metals function as catalysts in the contact-decomposition reaction. These metals are preferable because they have high melting points and can be heated by electric resistance heating. Of these metals, tungsten is particularly preferable. This is because tungsten has the highest melting point among all the metals, and therefore hydrogen gas can be very efficiently decomposed into radicals. Further, even if evaporated tungsten contaminates the silicon semiconductor, the characteristics of the TFTs are not degraded seriously insofar as the amount of the contaminant is little.
According to a fifth aspect of the invention, the method of the fourth aspect may further comprise, subsequent to the step of etching away the exposed portion of the second semiconductor thin film, forming a passivation film comprising a silicon nitride film in such a manner that the etched surface is not exposed to atmosphere.
When the surface of the channel is exposed to atmosphere, the surface contamination occurs, which causes the degradation of TFT characteristics. For this reason, it is preferable that the surface of the channel be protected by a passivation film so as not to be exposed to atmosphere. In the present invention, the channel etch may be performed by using a contact-decomposition reaction apparatus that can also be utilized for the formation of the passivation film. Thereby, the passivation film can be successively formed subsequent to the channel etch, and therefore the above-described method can be easily realized.
According to a sixth aspect of the invention, the method of the fifth aspect may be such that the first semiconductor thin film is a thin film comprising silicon; and the second semiconductor thin film is a thin film comprising silicon and an n-type impurity.
When the channel etch using an excited species is employed for a semiconductor employing a silicon thin film or a silicon thin film which contains an n-type impurity, the advantageous effects achievable by the present invention are more saliently exhibited. It is to be noted, however, that the present invention is not limited to the use of these semiconductor films. For example, a silicon-germanium thin film may also be used.
According to a seventh aspect of the invention, the method of the sixth aspect may be such that the silicon comprises amorphous silicon or polycrystalline silicon.
According to an eighth aspect of the invention, the method of the first aspect may be such that the non-ionic excited species is a non-ionic radical.
According to a ninth aspect of the invention, the method of the first aspect may further comprise, subsequent to the step of etching away the exposed portion of the second semiconductor thin film, forming a passivation film composed of a silicon nitride film in such a manner that the etched surface is not exposed to atmosphere.
According to a tenth aspect of the invention, the method of the second aspect may be such that the metal is selected from the group consisting of tungsten, tantalum, molybdenum, vanadium, platinum, and thorium, or is an alloy comprising at least two metals selected from the group consisting of tungsten, tantalum, molybdenum, vanadium, platinum, and thorium.
According to an 11th aspect of the invention, the method of the third aspect may be such that the molecules of the chemical substance comprise hydrogen, ammonia, or a mixture thereof.
These substances are easily decomposed and produce radicals when they make contact with a metal heated by electric resistance heating. Therefore, these substances are advantageous to efficiently perform channel etch.
According to a 12th aspect of the invention, the method of the third aspect may be such that the non-ionic radical is a hydrogen radical.
Hydrogen radicals do not etch such metals as aluminum (Al) and titanium (Ti) and therefore, the source/drain electrode metal made of Al, Ti, or the like can be utilized as a mask in the channel etch. For this reason, the above-described method is advantageous for increasing productivity in TFT fabrication. It is noted here that a technique of utilizing the source/drain electrode metal as a mask is disclosed in Japanese Examined Patent Publication No. 6-30397. This technique employs CH4 or NF3 for the etching gas and Cr for the source/drain electrode metal. The reason for the use of Cr is that, when a metal other than Cr is used for the source/drain electrode, the source/drain electrode is damaged by CH4 or NF3. The above-described method, on the other hand, employs hydrogen radicals, and therefore Ti and Al are usable for the source/drain electrode metal.
According to a 13th aspect of the invention, the method of the third aspect may be such that the non-ionic radical is a halogen radical.
The use of halogen radical is preferable in that, even when a native oxide film is present on the surface of the silicon thin film, which is a semiconductor thin film, halogen radicals are capable of easily etching the native oxide film. Therefore, the etching uniformity for silicon does not degrade.
According to a 14th aspect of the invention, the method of the 13th aspect may be such that the non-ionic halogen radical is a fluorine radical.
Fluorine radicals are desirable in that, for example, a desirable selective ratio is relatively easily obtained between metals, such as the ones used for the source/drain metal, and silicon, which forms the semiconductor thin film.
According to a 15th aspect of the invention, the method of the first aspect may be such that the step of etching is such that the excited species is generated in a microwave plasma generating chamber provided in isolation from an etching chamber in which the etching is performed, and from the generated excited species, only non-ionic excited species are selected and introduced into the etching chamber.
The excited species that are generated by a plasma generating apparatus contain ionic excited species, and the ionic excited species are accelerated by a direct current electric field component generated in the plasma generating apparatus. Therefore, when a plasma that contains ionic excited species is used for the channel etch, etching damage is caused to the channel. In the above-described method, however, only non-ionic excited species are selected to be used in the channel etch, and accordingly, the excited species are not accelerated by the direct current electric field component. Thus, etching damage is reduced.
According to a 16th aspect of the invention, the method of the 15th aspect may be such that the selected non-ionic excited species is a non-ionic radical.
According to a 17th aspect of the invention, the method of the 15th aspect may further comprise, subsequent to the step of etching away the exposed portion of the second semiconductor thin film, forming a passivation film comprising a silicon nitride film in such a manner that the etched surface is not exposed to atmosphere.
When the etched surface is exposed to atmosphere, the characteristics of the transistors are degraded by the contamination of the exposed surface. The above-described method prevents the surface from contamination, thereby improving production yields and reliability in product quality.
According to an 18th aspect of the invention, there is provided a method of producing a bottom gate thin film transistor, comprising the steps of:
forming a gate electrode on an insulating substrate;
forming a gate insulating film over the gate electrode;
forming a first semiconductor thin film for a channel over the gate insulating film;
forming a second semiconductor thin film for a source and a drain over the first semiconductor thin film;
processing stacked layers of the first semiconductor thin film and the second semiconductor thin film into an island;
subsequent to the step of processing stacked layers, depositing a source/drain electrode metal over the stacked layers;
etching a region of the deposited source/drain electrode metal, the region being located above the channel, in the depth direction to expose the second semiconductor thin film, whereby a source electrode and a drain electrode are formed; and
nitriding the exposed portion of the second semiconductor thin film using a non-ionic nitrogen-containing decomposition product that is produced by decomposing molecules of a chemical substance containing nitrogen atoms.
In the above-described method, a portion (nitrided region) of the second semiconductor thin film that is overlaid immediately above the channel is nitrided using a non-ionic nitrogen-containing decomposition product that is produced by decomposing the molecules of a chemical substance containing nitrogen atoms. In the nitriding step of the above-described method, unnecessary nitriding of the first semiconductor thin film for the channel, which lies below the second semiconductor thin film, is avoided, and only the region to be nitrided in the second semiconductor film is accurately nitrided. Therefore, good transistor characteristics are achieved. Moreover, the channel, which is formed in a layer below the nitrided region at the same time as the formation of the nitrided region, is protected by the nitrided film which is layered thereover without being exposed to atmosphere. Thus, the above-described method remarkably improves reliability and stability of TFTs.
Japanese Patent No. 3191745 discloses a method in which channel oxidizing or channel nitriding is used in place of a channel etch step of the bottom gate TFTs. More specifically, this publication discloses a method in which an amorphous silicon film made into an n-type is exposed in a plasma containing one of a) oxygen ions, b) both oxygen ions and nitrogen ions, and c) nitrogen radicals, to modify the n-type amorphous silicon film into an insulating film of an oxynitrided film. However, it is known that in cases where the active layer is made of a-Si:H, the use of oxidizing degrades the transistor characteristics below the threshold voltage. Therefore, nitriding is advantageous over oxidizing. Nevertheless, the nitriding technique described in the foregoing publication uses a plasma, and for this reason, this technique cannot obtain sufficient rate of nitriding. Moreover, a plasma contains a ionic decomposition product. The ionic decomposition product is accelerated by electric field and collides with the semiconductor film. Thereby, the silicon film placed underneath the n-type amorphous silicon film is damaged, and consequently, the transistor characteristics are degraded. In contrast, the above-described method of the present invention uses a non-ionic nitrogen-containing decomposition product, and therefore, damage to the back channel is little. Hence, the above-described method achieves stable transistor characteristics.
According to a 19th aspect of the invention, the method of the 18th aspect may be such that the non-ionic nitrogen-containing decomposition product is generated by bringing a metal heated by electric resistance heating into contact with the molecules of the chemical substance containing nitrogen atoms.
This method achieves efficient generation of the non-ionic nitrogen-containing decomposition product and thereby improves productivity.
According to a 20th aspect of the invention, the method of the 19th aspect may be such that the molecules of the chemical substance comprise ammonia.
When the molecules of the chemical substance comprise ammonia, the non-ionic nitrogen-containing decomposition product is efficiently generated by contact-decomposition reaction with the metal heated by electric resistance heating. As a result, the nitriding proceeds swiftly.
According to a 21st aspect of the invention, the method of the 19th aspect may be such that the metal is selected from the group consisting of tungsten, tantalum, molybdenum, vanadium, platinum, and thorium, or is an alloy comprising at least two metals selected from the group consisting of tungsten, tantalum, molybdenum, vanadium, platinum, and thorium.
These metals function as an excellent catalyst for the contact-decomposition reaction. These metals are preferable because they have high melting points and can be heated by electric resistance heating. Of these metals, tungsten is particularly preferable. This is because tungsten has the highest melting point among all the metals, and therefore, the molecules of the chemical substance containing nitrogen atoms are very efficiently decomposed into radicals. Further, even if evaporated tungsten contaminates the silicon semiconductor, the characteristics of the TFTs are not degraded seriously insofar as the amount of the contaminant is little.
According to a 22nd aspect of the invention, the method of the 19th aspect may be such that the first semiconductor thin film is a thin film comprising silicon; and the second semiconductor thin film is a thin film comprising silicon and an n-type impurity.
It is preferable that the channel nitriding using the non-ionic nitrogen-containing decomposition product is applied to a semiconductor film comprising silicon, because the advantageous effects achievable by the present invention are thereby more saliently exhibited.
According to a 23rd aspect of the invention, the method of the 22nd aspect may be such that the silicon comprises amorphous silicon or polycrystalline silicon.
The channel nitriding using the non-ionic nitrogen-containing decomposition product exhibits more salient advantageous effects when applied to a silicon thin film comprising amorphous silicon or polycrystalline silicon.
According to a 24th aspect of the invention, there is provided a bottom gate thin film transistor comprising:
a gate electrode formed on an insulating substrate;
a gate insulating film formed over the gate electrode;
a channel region comprising a first semiconductor thin film stacked over the gate insulating film;
a source region and a drain region each comprising a second semiconductor thin film that is stacked over a region of the first semiconductor thin film exclusive of the channel region;
a source electrode and a drain electrode formed on the second semiconductor thin film; and
a passivation film composed of a silicon nitride film formed on the channel;
wherein a portion of the channel contains at least one element selected from the group consisting of tungsten, tantalum, molybdenum, vanadium, platinum, and thorium, the portion of the channel being adjacent to a surface thereof which is in contact with the silicon nitride film, and the total atomic density of the at least one element is in the range of from 1xc3x971016xc2x7cmxe2x88x923 to 1xc3x971019xc2x7cmxe2x88x923.
The present inventors have found that when non-ionic excited species such as radicals are generated, the metal heated by electric resistance heating is evaporated and the evaporated metal, though in a trace amount, contaminates the first semiconductor thin film, which forms the channel. However, the present inventors have also found that when the metal is at least a metal selected from the group of tungsten, tantalum, molybdenum, vanadium, platinum, and thorium, adverse effects of the contaminant on transistor characteristics are negligible insofar as the amount of the contaminant is an atomic density of from 1xc3x971016xc2x7cmxe2x88x923 to 1xc3x971019xc2x7cmxe2x88x923. Based on these findings, the above-described method of the invention was thus accomplished.
According to a 25th aspect of the invention, the bottom gate thin film transistor of the 22nd aspect may be such that the first semiconductor thin film is a thin film comprising silicon; and the second semiconductor thin film is a thin film comprising silicon and an n-type impurity.
According to a 26th aspect of the invention, the bottom gate thin film transistor of the 25th aspect may be such that the silicon comprises amorphous silicon or polycrystalline silicon.
According to a 27th aspect of the invention, there is provided a bottom gate thin film transistor comprising:
a gate electrode formed on an insulating substrate;
a gate insulating film formed over the gate electrode;
a channel formed of a first semiconductor thin film stacked over the gate insulating film;
a source and a drain each formed of a second semiconductor thin film stacked over the first semiconductor thin film;
a nitrided region in which a portion of the second semiconductor thin film disposed immediately above the channel is nitrided; and
a source electrode and a drain electrode, each formed on a portion of the second semiconductor thin film exclusive of the nitrided region;
wherein a portion of the channel contains at least one element selected from the group consisting of tungsten, tantalum, molybdenum, vanadium, platinum, and thorium, the portion of the channel being adjacent to a surface thereof which faces the nitrided region; and the total atomic density of the at least one element is in the range of from 1xc3x971016xc2x7cmxe2x88x923 to 1xc3x971019xc2x7cmxe2x88x923.
According to a 28th aspect of the invention, the bottom gate thin film transistor of the 27th aspect may be such that the first semiconductor thin film is a thin film comprising silicon; and the second semiconductor thin film is a thin film comprising silicon and an n-type impurity.
According to a 29th aspect of the invention, the bottom gate thin film transistor of the 28th aspect may be such that the silicon comprises amorphous silicon or polycrystalline silicon.
According to a 30th aspect of the invention, there is provided a liquid crystal display device comprising:
a first substrate comprising a plurality of scan electrodes, a plurality of data electrodes intersecting the scan electrodes, a plurality of thin film transistors provided at the intersectional positions of the scan electrodes and the data electrodes so that at least one of the plurality of thin film transistors is provided at each of the intersectional positions, and a plurality of pixel electrodes connected to the thin film transistors;
a second substrate comprising a counter electrode opposed to the pixel electrodes; and
a liquid crystal sandwiched between the first substrate and the second substrate;
wherein each of the thin film transistors is a bottom gate thin film transistor according to any one of the foregoing 24th to 29th aspects of the invention.
Any of the bottom gate thin film transistors described in the above 24th to 29th aspects of the invention sufficiently exhibit the characteristics when used for the TFTs in a liquid crystal display device. Therefore, the above-described configuration achieves a liquid crystal display device that is excellent in terms of stable operation.
According to a 31 aspect of the invention, there is provided an organic electroluminescent display device comprising:
a first substrate comprising a plurality of scan electrodes, a plurality of data electrodes intersecting the scan electrodes, a plurality of thin film transistors provided at the intersectional positions of the scan electrodes and the data electrodes so that at least one of the thin film transistors is provided at each of the intersectional positions, and a plurality of pixel electrodes connected to the thin film transistors;
a second substrate comprising a counter electrode opposed to the pixel electrodes; and
layer comprising an organic electroluminescent material, the lay being sandwiched between the first substrate and the second substrate;
wherein each of the thin film transistors is a bottom gate thin film transistor according to any one of th e foregoing 24th to 29th aspects of the invention.
Any of the bottom gate thin film transistors described in the above 24th to 29th aspects of the invention sufficiently exhibit the characteristics when used for switching elements in an organic electroluminescent display device. Therefore, the above-described configuration achieves an organic electroluminescent display device that is excellent in terms of stable operation.